Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Often, such injection devices are required to work with sealed hypodermic syringes which typically have a hermetically sealed cover or “boot” that covers the hypodermic needle and maintains the sterility of the syringe contents. The boot may be formed from a rubber material. Naturally, it is necessary to maintain the sterility of the syringe contents up to the point of administration, which means that the boot must be removed with the syringe inside the injection device.
Typically, the action required to remove the boot from the syringe entails either pulling the boot away from the syringe or twisting the boot and pulling it away from the syringe. If a filled syringe has been around for quite a while before it is used, it is often difficult to remove the boot from the syringe; a substantial force is often required.
The injection device may be designed so that removal of a cap of the injection device also removes the boot from the syringe. In such cases, the boot must be connected to the cap in such a way that the force required to remove the boot from the syringe is less than the force required to disconnect the boot from the cap. In order to connect the boot to the cap, an “insertion” force is exerted on the syringe. If this insertion force is too high, damage to the syringe and/or boot will occur, and the injection device will fail to operate correctly. The force required to remove a boot from a syringe is generally an order of magnitude greater than the maximum insertion force that can be exerted on the syringe via the boot without causing damage to the boot and/or syringe.
Therefore, there is a need for a means for connecting a cap of an injection device to a boot of a syringe in such a way that removal of the cap from the injection device causes removal of the boot from the syringe, and connection of the cap to the boot exerts a minimal force on the syringe and/or boot.